In 1967, having made the decision not to enter the priesthood,[4] Barthelemy returned to New Orleans and became an administrative assistant in the office of Total Community Action. In 1968, Barthelemy married Michaele Thibideau.[5] From 1969 to 1972 he was director of the Parent Child Center of Family Health, Inc. During these years he also completed a Master of Social Work degree at Tulane University in New Orleans, worked part-time for the Urban League of Greater New Orleans and assisted with various political campaigns, joining COUP, a political organization based in the 7th Ward of New Orleans.[6]

From 1972 to 1974 Barthelemy was Director of the Department of Welfare under Mayor Moon Landrieu. Backed by COUP, Barthelemy was elected in 1974 to one term in the Louisiana State Senate from District 4;[7] he was the first African-American to serve in that body since Reconstruction. While he served in the Legislature, he also joined Xavier University as assistant director of the Urbinvolve Program and as an instructor in the Department of Sociology and became an adjunct faculty member in the Applied Health Sciences Department, Maternal and Child Health Section, of Tulane University.

In 1978 Barthelemy was elected to an at-large seat on the New Orleans City Council, a position he held for two terms.[8] While in the council, Barthelemy become known for his longstanding rivalry with Mayor Ernest "Dutch" Morial. In 1979, Barthelemy was elected to be in state Senate, beating out a sixteen year, white incumbent [9] He defeated Bill Jefferson in the 1986 mayoral election to succeed Morial. On May 5, 1986, Barthelemy began his first term as mayor of New Orleans.[10]

Sidney Barthelemy first set his sights on becoming mayor of New Orleans in 1982, when he was reelected to the city council.[11] Barthelemy seemed the complete opposite of his fiery predecessor Ernest Morial, who had something of a "will of iron".[12] Barthelemy was more relaxed in demeanor. Not allowed to run for a third term as mayor, Morial supported Barthelemy's runoff opponent William J. Jefferson.[13] Eliminated in the primary was former state Representative Sam A. LeBlanc, III, who finished with a strong 25 percent of the vote. The runoff between Barthelemy and Jefferson was the first in which both contenders were African Americans.[14] Many African Americans felt as though Barthelemy was concentrating his focus on white-collar businesses.[15] This contributed to the dramatic shift in support from black to white. Barthelemy received 58 percent of the total votes but only 43 percent among blacks. He won 86 percent from the dwindling number of white voters in the city.[16] Oddly, in contrast to 1986, Barthelemy won reelection in 1990 with 86 percent of the black vote but only 23 percent from whites.[17]

First elected in 1986, Barthelemy became the second African American mayor of New Orleans with 58% of the vote.[18] Barthelemy's mayoralty began under difficult circumstances. Federal government revenue sharing to municipalities had been progressively reduced from 1981 onwards, and had ended by the time Barthelemy took office in 1986. A regional economic slump, the so-called Oil Bust added to these budget woes. Many federal and state funds decreased from around forty million dollars to less than six million.[19] Louisiana's energy-dependent economy, already slowing as the price of oil declined from its 1980 high point, was pummeled in early 1986 by a sudden collapse in price per barrel from over $27 to less than $10. With the economy and this financial crisis, the unemployment rate in the city was over ten percent [20] and the deficit was $30 million.[21] To solve this, Barthelemy worked with the city council and developed a plan to dig the city out of the deficit. This plan involved cutting costs, raising fees, and privatizing operations, in the process of which over 1000 workers lost jobs.[22]

Taking a less hands-on approach to economic development than his predecessor, Barthelemy preferred to let the private sector be the primary engine of growth which bothered many people.[23] Though Barthelemy took a pro-business stance, his unfocused, laissez-faire approach to attacking the city's problems was harmful in the post-Oil Bust period; it was during his and his successor's mayoralties that New Orleans lost its remaining attractiveness as a center for corporate white collar employment. This shift towards supporting the business community caused a complex racial issue as well, as many blacks began to feel as if Barthelemy was turning his back on them.[24] As recently as the late 1970s and early 1980s, downtown New Orleans had experienced a building boom, with multiple office towers constructed to house the headquarters, or large regional offices, for companies such as Freeport-McMoRan, Pan American Life Insurance, Exxon, Chevron, Gulf Oil, Amoco, Mobil, Murphy Oil and Texaco. In the mid-80s these firms, along with other large employers such as Royal Dutch Shell, Louisiana Land and Exploration and McDermott International, employed thousands of white collar workers downtown, with thousands more employed by others providing services to them. Only Shell and Pan American Life Insurance remain as significant entities in New Orleans today. Though Barthelemy probably couldn't have averted the consolidation of Big Oil white collar employment to Houston, he was unsuccessful in creating an economic climate sufficiently conducive to the growth of replacement white collar employers downtown, or elsewhere in the city. A big disappointment occurred in 1988, when defense and space contractor Martin Marietta lost its bid to construct the crew modules for NASA's Space Station Freedom. Martin Marietta would have fabricated the space station hardware at its Michoud Assembly Facility in Eastern New Orleans, already the manufacturing location of the Space Shuttle's external tank. The contract, won by Boeing, resulted in thousands of jobs created at that company's chosen assembly site, Huntsville, Alabama. More disappointment followed the creation of the Metrovision regional economic development partnership, as that body largely failed to diversify or attract additional investment to Metro New Orleans' economy.

Barthelemy's administration was not without economic development successes, as he managed to bring additional investment to New Orleans East, most notably by attracting the giant Pick 'n Save distribution center to the New Orleans Regional Business Park (then known as the Almonaster-Michoud Industrial District). The Pick 'n Save project embodied the city's renewed efforts to leverage the existing infrastructure of the Port of New Orleans, then experiencing a resurgence under the leadership of Ron Brinson, by attracting modern warehousing and distribution facilities to the city. In the wake of the Oil Bust, however, the Barthelemy administration most forcefully advocated for the continued development of New Orleans' tourist and convention industry. Tourism was the only sector of the city's economy to exhibit meaningful growth under Barthelemy. Several high-profile wins occurred, including attracting the Republican National Convention to the city in 1988 and the NCAA Final Four tournament in 1993. Barthelemy also oversaw the opening of the Aquarium of the Americas, the Riverfront streetcar line, the New Orleans Centre and Riverwalk downtown malls, and encouraged the first expansion of the Ernest N. Morial Convention Center (then known as the New Orleans Convention Center).[25]

In administering city government, Barthelemy managed to gradually eliminate the $30 million budget deficit he inherited in 1986, but his methods of raising revenue - attempting to impose an "earnings tax" on the personal income of suburbanites who worked within the city limits, legalization of a land-based casino and riverboat gambling - were controversial. He took control of agencies such as the Housing Authority of New Orleans (HANO) [26] and the Regional Transit Authority (RTA), providing appointments to members of COUP and other political allies.

Other highlights of the Barthelemy administration were the visit of Pope John Paul II in 1987, the passage of the controversial "anti-discrimination" ordinance affecting the membership in Carnival krewes, an unsuccessful, city-sponsored effort to redevelop Louis Armstrong Park into a Tivoli Gardens-style recreation park/amusement center, an unsuccessful proposal to construct a new international airport in New Orleans East, in what later became the Bayou Sauvage National Wildlife Refuge, the post-World's Fair gentrification and redevelopment of the downtown Warehouse District, and the securing of funding for a new sports arena next to the Superdome. Consistent with the city's increasing economic tilt to tourism and the cultural economy, Barthelemy's mayoralty also supported a large addition to the New Orleans Museum of Art, in which the museum attained its present size, as well as the creation of the Louisiana Children's Museum in the Warehouse District. Plans to re-use portions of the former Canadian Pavilion of the World's Fair, including its IMAX theater, as a new Louisiana Science Museum came to nothing, however.

In April 1993 Barthelemy succeeded in accomplishing two notable things. He destroyed the old Canal streetcar barn, listed on the National Register of Historic Places as probably the oldest surviving carbarn in the country, and assisted the US Department of Transportation in destroying section 4(f) of the Transportation Act of 1966, one of the oldest and most important environmental and historic preservation laws in the country, a law that could have protected the carbarn.[27]

In 1993 Barthelemy was responsible for bringing a little-known statewide political perk to the attention of the state's voters. At his son's graduation ceremony from Brother Martin High School, it was announced that he would be receiving the "Mayor's Academic Scholarship to Tulane University." This drew a mixed reaction from the crowd in attendance. The next day, the story was featured on the local news in New Orleans. By the end of the week, the story was the talk of the state, and it was revealed that in addition to the Mayor, all members of the Louisiana Legislature could grant scholarships to Tulane University. Louisiana legislators were permitted to award the scholarships to anyone they saw fit, provided that the nominee was a bona fide citizen and resident of the district or parish and "shall comply with the requirements for admission established"[28] by the university's Board. Through further investigation, it was revealed that politicians regularly gave these scholarships to their own family members and to the children of political allies. In some instances, the scholarships were given in exchange for political favors.

In the wake of these revelations, Mayor Barthelemy and four state lawmakers, RepublicansJim Donelon, Kernan "Skip" Hand, and Ken Hollis and the Democrat Steve Theriot, all from the New Orleans suburbs, admitted to having awarded Tulane Universityscholarships to family members. Specifically, it was learned that state officials were able to request a tuition waiver — worth some $17,000 in 1993 - to one student per year under an 1884 state law approving the conversion of the public University of Louisiana into a private institution, Tulane University, upon the school's receipt of Paul Tulane's and Josephine Newcomb's bequests.[29]

Throughout his political career, Barthelemy carried a reputation as a quiet and mild-tempered politician,[30] in marked contrast to his fiery predecessor Dutch Morial, and to typical New Orleans politicians in general. His critics interpreted this as a sign of passivity and poor leadership. Under his mayoralty, the city's population declined significantly, the crime rate increased dramatically, the murder rate increasing along with the amount of drug use.[31] The performance of the city's public school system continued to erode as well - though in fairness the schools were controlled by the Orleans Parish School Board. National trends were also unfavorable to cities during Barthelemy's mayoralty; New Orleans was far from alone in grappling with economic stagnation and rising crime. Still, many of the alleged deficiencies of the Barthelemy administration were already acknowledged as fact by the public by the end of Barthelemy's first term. His re-election campaign in 1990 was marked by widespread criticism of his administration, and of his perceived lack of leadership - but Barthelemy managed to defeat his challenger Donald Mintz in a runoff election in which Barthelemy received 55% of the vote.[32]

Fritz Windhorst, who served as a Republican member of the Louisiana State Senate from Jefferson and Orleans parishes from 1972 to 1992, described Barthelemy this way: "Sidney doesn’t whine or complain when things go badly…He doesn't threaten people who cross him. Just having him as mayor has sharply reduced the anti-New Orleans feelings in the legislature."[33] This calm demeanor Barthelemy himself assessed in a quote where he stated, " In a quote from Barthelemy, he remarks that, "People tend to underestimate me because I don't bang my fist on the table and jump up and scream, but I know how to get the job done and that's always been my aim—to get things done.".[34]

Although Barthelemy was the subject of much criticism during his time as mayor, recently he has been getting some praise from the city for his efforts in attaining the Audubon Aquarium of the America, the sports arena next to the super dome (even though this was finished three years after his term as mayor ended), and the redevelopment of the D.H. Holmes store.[35] Council woman Cynthia Hedge Morrell recently remarked during an event at Gallier Hall, ""The city of New Orleans would not be where it is today if those tough decisions were not made and you had not had the fortitude to endure the wrath of the media and the wrath of people who did not understand" that "good decisions were being made for the future of our city".[36] Currently, Sidney Barthelemy is serving as the Director of Governmental Affairs for Historic Restoration, Inc., a real estate development group based in New Orleans. On January 7, 2006, Barthelemy joined former mayors Moon Landrieu and Marc Morial in a meeting with parish presidents from the New Orleans metropolitan area to discuss post-Katrina plans for regional flood protection [2].[37]